The present invention relates to a secondary-electron multiplier adapted to be incorporated in a secondary-electron multiplier section of a mass spectrometer, ion detector, photomultipler or image intensifier in order to attain a stable and simple amplification of the electrons produced as the result of the striking of the photons or charged particles on a secondary-electron emission layer.
The typical prior art secondary-electron multipliers are of the venetian blind type, box and grip type, etc., in which the dynodes are arranged in more than ten stages. They are of course very complex in construction, and many problems must be solved before they may be made compact in size and light in weight. Furthermore, a bank of resistors for dividing the voltage to be applied to the individual dynodes must be provided without the secondary-electron multiplier proper. The secondary-electron multipliers which are incorporated into the charged particle detectors, photon counters, camera tubes and image intersifiers which in turn are mounted on the rockets and satellites, must be small in size, light in weight yet very reliable in operation with the high gain. For this purpose, there has been devised and demonstrated a secondary-electron multiplier of the type in which the group of separate dynodes is replaced by a continuous high resistance surface which has the double function of emitting the secondary-electrons and dividing the voltage. The secondary-electron multiplier of the type described above is called the channel type secondary-electron multiplier, which may be further divided into (a) a parallel plate type secondary-electron multiplier and (b) a pipe type secondary electron multiplier.
The channel type is very simple in construction as compared with the separate dynode type, and is also very easy to operate because it has only two terminals. Furthermore, the channel may be reduced in size independently of the gain as long as the ratio a = l/d is maintained constant, where l = length of channel and d multipliers diameter or spacing. Therefore, channel type secondary-electron multiliers may be made compact in size and light in weight, and still may have a high gain by increasing the voltage to be applied.
In the pipe type secondary-electron multipliers now available in the market, a glass or ceramic tube with a very small diameter is bowed or curved in the form of a spiral in order to prevent ion feedback. In general, the pipe type secondary-electron multipliers must be considerably elongated in length in order to improve the gain.
In the secondary-electron multipliers, the secondary electrons produced under applying the DC voltage are further accelerated and multiplied. For this purpose, the electro-conductive layer having the specific resistance of the order of 10.sup.8 - 10.sup.10 ohm-cm must be provided. When the specific resistance is low, Joule heat is generated due to the high electric field applied to the secondary-electron multiplier in order to accelerate the electrons. On the other hand, when the specific resistance exceeds 10.sup.10 ohm-cm, the conductive layer becomes an insulating layer so that the portion where the secondary electrons are produced is positively charged. As a result, the supply of electrons is interrupted, and the conductive layer is consequently positively charged. Therefore, the conductive layer must have the specific resistance or volume resistivity of the order of 10.sup.5 - 10.sup.10 ohm-cm.
So far in the inorganic materials only the vacuum evaporation process, the metal plating process or sputtering process was employed in order to form the layer. Except for some ion crystals which are not capable of forming a film or the like, they cannot be dissolved in a solvent, so that it has been impossible to provide an inorganic paint for forming the secondary-electron emission layer. Thus, it has been extremely difficult to coat the inner surface of a fine-diameter tube of the secondary-electron multipliers to form the secondary-electron emission layer. Therefore, there has been devised and demonstrated a method for reducing a lead glass tube in the hydrogen flow, thereby forming the lead conductive layer upon the inner surface of the tube. Furthermore, there has been also devised and demonstrated a bulk type secondary-electron multiplier of the type in which a tube is made of ceramic such as BaTiO.sub.3 or ZnTiO.sub.3. The glass and bulk type secondary-electron multipliers are difficult to fabricate and easily susceptible to damages under the mechanical shock or impact. Therefore, they cannot be incorporated in the detectors, image intensifiers or the like to be mounted upon the rockets and satellites which are subjected to considerably strong shock, impact and vibration.
In order to eliminate the above described problems, there have been devised and demonstrated secondary-electron multipliers of the type making use of the secondary-electron emission capability of the electron-conductive polymeric compositions, as disclosed for instance in Canada Pat. No. 883443 and Rev. Sci. Inst. 40(9) 1239 (1969). They are the bulk and flexible channel type, wherein the secondary-electron multipliers are provided by molding the electron-conductive polymeric compositions by making full use of the suitable molding properties flexibility of the high-polymer materials.